1. Field of the Invention
This invention relates to an image sensing apparatus in which as an object image moves on the image sensing plane, a focus detecting area, or the like is made to continue being adjusted to the object by tracing (tracking) the moving of the object image.
2. Description of the Related Art
In the imaging instruments such as video cameras, it has been known to use a high frequency component of the video signal in detecting the degree of sharpness of an object image at the image sensing plane, by which the position of a focusing lens is controlled so as to maximize the magnitude of that high frequency component, so that the camera is automatically controlled to come into an in-focus state. In more detail, the video signal changes rapidly at or near the boundary line of the object with the result that its high frequency component increases. It is proven that under the condition of increasing of its high frequency component, the higher the level of the video signal, the nearer the focusing lens lies to the in-focus state.
When shooting an object, if, as the object is moving, or the camera is moving, the object appears to move relative to the camera, the high frequency component will be lowered by that motion, or if the object goes out of the focus detecting area, the automatic focus adjusting device will be caused to focus on another object. To overcome such very demanding problems, there has been a previous proposal for making the focus detecting area trace (track) the moving object as long as motion of the object is detected, so that despite the motion of the object, the automatic tracing device enables the focusing lens to continue focusing on the aimed object.
In the prior-known tracing devices, many methods have been proposed, for example, in which the color, luminance, or other feature, sampled at a moment, of the object is memorized and then compared with that sampled at the next moment to detect whether or not the object has moved. Another method is that the comparison is made in vertical and horizontal correlation with respect to different times.
However, these methods have many problems in that many situations where the feature of the object cannot accurately be detected are encountered, in that the construction of the device becomes complicated, and in that the required size of the memory is relatively large.
To overcome those problems, a method has been proposed in which a designated area is set on the image sensing plane, then the difference in the average video signal level between the outside and the inside of the designated area is determined, and then the focus detecting area is moved so that the difference reaches the maximum. The use of this method simplifies the structure of construction, thus achieving a great advance in the art.
From later experiments and investigation, however, it has been found out that to apply to actual practice the above-described object tracing method based on the average level difference in the video signal between the outside and the inside of the designated area, there is much room for further improvement.
That is, in a situation where the luminance difference between the object and its background is small, the object tracing device is very susceptible to the influence of another object. Hence, the accuracy and reliability of detection for motion of the object is so lowered that sufficient ability to keep focusing on the aimed object cannot be obtained at any speed of movement of the object.
Meanwhile, in the video camera, the focusing position of the photographic lens is controlled by a method in which the degree of sharpness of the optical image on the image sensing plane is determined by the high frequency component of the output video signal, and then a maximum value of the high frequency component is sought, and the size of aperture opening of the iris is controlled by a method in which the level of average luminance of the output video signal is used. In these methods, all the video signals over the entire area of the picture frame are required to be processed. Differing from these methods, there have been previous proposals for a spot method of distance and light measurement (as in, for example, Japanese Patent Applications Nos. Sho 62-277382 and 62-277384) wherein the object detecting area is moved on the image sensing plane so as to maximize the difference in the average video signal level between the inside and the outside of a designated area on the image sensing plane, and a distance measuring area or a light measuring area is set on the basis of the video signal from that object detecting area.
The above-described spot method relies on the general premise that the level of the video signal for the object differs largely from the level of the video signal for its background. When the difference in video signal level between the object and its background is small, a problem will arise in which the accuracy of tracing control to the moving object is very much lowered.
Patent applications related to the invention are as follows:
On the tracing (tracking), there are U.S. patent applications Ser. Nos.:
737,163 filed on May 23, 1985 (tracing by extracting a feature point); PA1 106,427 filed on Oct. 8, 1987 (peak tracing); PA1 240,915 filed on Sep. 6, 1988 (tracing by luminance difference); and PA1 237,511 filed on Aug. 26, 1988 (detecting of motion of the image). PA1 017,183 filed on Feb. 19, 1987; PA1 046,252 filed on May 5, 1987; PA1 107,451 filed on Oct. 8, 1987; PA1 121,624 filed on Nov. 17, 1987; PA1 154,078 filed on Feb. 9, 1988; and PA1 154,795 filed on Feb. 11, 1988.
As another example of application of the tracing technique to the automatic exposure apparatus, there is U.S. patent application Ser. No. 264,204 filed on Oct. 28, 1988.
Further, on the general case of auto focusing, there are U.S. Pat. No. 4,762,986 and U.S. patent applications Ser. Nos.: